The present invention relates to the enhancement of the heat transfer properties of surfaces used in heat exchangers. Applicants have found that by a novel chemical etching procedure, the formation of a particular surface topography will enhance the heat transfer properties exhibited by various heat exchangers.
The development of high performance nucleate boiling surfaces for commercial use in heat exchangers has been the focus of considerable industrial research efforts over the last several decades. Proposed techniques for promoting nucleate boiling include the following:
(1) Abrasive treatment--Abrasively roughening the surface of a plate will at least temporarily improve nucleate boiling, a phenomenon that has been known for many years.
(2) Inscribing often grooves--Forming parallel grooves by sharp pointed scribes, with a scratch spacing of 2 to 2.5 bubble diameters was found to increase the boiling coefficient of a copper plate, as reported by Bonilla, C.F. et al. in "Pool Boiling Heat Transfer From Grooved Surfaces", Chem. Eng. Prog. Supp. Ser., vol. 61, No. 57, pp 280-288 (1965).
(3) Forming three dimensional cavities--Pressing cylindrical or conical cavities into a copper surface was found to significantly enhance boiling performance. It was found that the "re-entrant" type cavities were superior as a vapor trap. See, for example, Benjamin, J.E. et al., "Possible Growth in Nucleate Boiling a Binary Mixture", International Developments in Heat Transfer, ASME, New York, 1961, pp 212-218.
(4) Electroplating--Electroplating layers of certain coating materials such as copper at very high current densities, causing the formation of a porous coating on the surface, was disclosed on producing a large heat transfer increase in U.S. Pat. No. 4,018,264 issued to Albertson in 1977.
(5) Chemical etching--Exposing the surface of a wall to an etching bath for a short period of time was found to substantially improve the heat transfer properties of the wall, as disclosed in U.S. Pat. No. 4,360,058 issued to Muellejans in 1982.
None of the prior art approaches to enhancing heat transfer performance is fully satisfactory. For example, the formation of discrete cavities by mechanical treatment is difficult and expensive. Furthermore, mechanical treatment as well as electroplating may be impractical on thin metal walls. Furthermore, mechanical treatment is generally not amenable to the relatively inaccessible walls of plate and fin heat exchangers.
Heat transfer enhancement is especially desirable in the reboiler/condenser system of a conventional air separation plant, which involves boiling oxygen at low pressure on one side of an aluminum divider and condensing nitrogen at high pressure on the other side. The efficiency of such a system is limited by the heat transfer between the aluminum divider and the boiling oxygen. An improvement in heat transfer would result in savings in energy costs by reducing the pressure requirements for the nitrogen or in initial equipment costs by reducing the dimensions of the system.
It is therefore a principal object of the present invention to enhance the heat transfer of a heat exchanger surface by the formation of a surface topography which promotes rapid and stable nucleate boiling.
It is yet another object of the present invention to enhance the heat transfer properties of a heat exchanger surface utilizing a chemical etching process which is simple and economical.
It is a further object of the present invention to promote nucleate boiling without the necessity of highly involved or expensive mechanical treatment which cannot be applied to inaccessible walls.
It is a further object of the present invention to facilitate heat transfer during the phase change of a fluid, for example, during cryogenic distillation of a permanent gas.
It is yet a further object of the present invention to enhance the heat transfer properties of a heat exchange surface when boiling liquids of low surface tension such as cryogenic nitrogen or oxygen.
It is yet a further object of the invention to enhance the heat transfer properties of a heat exchange surface in contact with water or refrigerants such as freon or ammonia.
It is yet another object of the invention to accomplish a process for enhancing the heat exchange properties of a surface which is practical for plate and fin type heat exchangers, for example, the inner or outer surface of a shell and tube heat exchanger.
It is a further object of the invention to improve heat exchangers by procedures compatible with existing fabrication processes.